Tom visits George at his ranch. Although a novice on horseback, Tom decides to join George in a morning ride. George recommends that Tom ride an unbroken stallion named Satan. While mounting Satan, Tom is thrown headlong through the windshield of Tom's car. The damage to the car is $2,000 and Tom's own insurance pays him $1,500 ($500 deductible). Under what principle could Tom's insurance company sue George for his negligence?